The present invention relates to a method for ejecting liquid droplets onto various media, such as a sheet of paper, to record images on the medium. In particular, it relates to a method for ejecting extremely fine liquid droplets.
There are various recording methods which have been put to practical use in various printers or similar apparatuses. Among them, the recording methods which employ the ink jet systems disclosed in the specifications of U.S. Pat. Nos. 4,723,129, and 4,740,796 are very effective. According to these patents, thermal energy is used to cause so-called xe2x80x9cfilm boilingxe2x80x9d, and the bubbles generated by the xe2x80x9cfilm-boilingxe2x80x9dare used for ejecting liquid in the form of droplets.
Among the ink jet based recording methods, the one disclosed in the specification of U.S. Pat. No. 4,410,899 has been known as an ink jet system based recording method of a sort that does not block a liquid path while forming a bubble.
The inventions disclosed in the above documents are applicable to various recording apparatuses. However, there is no record that a recording system which allows a bubble that is formed in an ink path to eject liquid, to become connected to the atmospheric air (hereinafter, xe2x80x9cbubble-atmospheric air connection systemxe2x80x9dor simply, xe2x80x9cbubble-air connection systemxe2x80x9d), has been developed enough to be put to practical use.
The conventional xe2x80x9cbubble-air integration systemsxe2x80x9d rely on bubble explosion, but they are not stable in terms of liquid ejection. Therefore, they cannot be put to practical use. However, there is a promising system, which is disclosed in Japanese Laid-Open Patent Application No. 161935/1979. The liquid ejection principle in this system is unclear. According to this system, a cylindrical heater is fitted in a cylindrical nozzle, and the liquid in the nozzle is separated into two portions by the bubble formed in the nozzle. However, this system also has a problem that a large number of ultramicroscopic liquid droplets are generated at the same time as a primary liquid droplet is generated.
The specification of U.S. Pat. No. 4,638,337 also presents a structure of the bubble-air integration system, in its Prior Art section. However, this patent presents this structure, in which a bubble generated in liquid by the thermal energy given by a heat generating element becomes connected to the atmospheric air, as an undesirable example of the liquid ejection head structure in which ink fails to be ejected or ink is ejected in a direction deviating from the predetermined direction.
This phenomenon occurs under a specific abnormal condition. For example, if a bubble, which has been grown by the driving of a heat generating element, ejects liquid at a point in time when the meniscus, which is desired to be located adjacent to the ejection orifice of an ink path (nozzle) at the moment of ink ejection, has just retracted toward the heat generating element, the liquid, or the ink, is ejected in an undesirable manner.
This is evident because this phenomenon is clearly described, as an undesirable example, in the specification of U.S. Pat. No. 4,638,337.
On the other hand, examples of practical application of the bubble-air connection system are disclosed in Japanese Laid-Open patent applications Nos. 10940/1992, 10941/1992, 10942/1992 and 12859/1992. These inventions disclosed in Japanese official gazettes resulted from the pursuit of the causes of the generation of the aforementioned liquid splashes or ink splashes by bubble explosion, and the unreliable bubble formation. They are recording methods which comprise a process in which thermal energy is given to the liquid in a liquid path in an amount large enough to cause the liquid temperature to suddenly rise to a point at which so-called xe2x80x9cfilm boilingxe2x80x9d of the liquid occurs and a bubble is generated in the liquid in the liquid path, and a process in which the bubble generated in the recording process becomes connected to the atmospheric air.
According to these recording methods, which cause a bubble to become connected to the atmospheric air adjacently to the ejection orifice of the liquid path, liquid can be desirably ejected in response to a recording signal without causing the splashing of liquid or formation of liquid mist, which is liable to occur in the case of a conventional printer or the like, adjacently to ejection orifices.
From the viewpoint of the uniformity with which a bubble grows and becomes connected with the atmospheric air, in other words, from the viewpoint of reliability in liquid ejection accuracy, the aforementioned bubble-air connection liquid ejection method is desired to be used with a so-called side shooter type liquid ejection head, in which ejection orifices are positioned to directly face corresponding electrothermal transducers.
However, as a liquid droplet ejected from the aforementioned side shooter type liquid ejection head is reduced in volume to form an image of higher quality, the way a bubble becomes connected to the atmospheric air affects the direction in which a liquid droplet is ejected. In particular if the volume of a liquid droplet is reduced to no more than 20xc3x9710xe2x88x9215m3, the trailing portion (portion which connects the primary-droplet-to-be portion to the liquid path), and the satellite liquid droplets generated by the trailing portion, affect image quality. In addition, the smaller the liquid droplet volume, the higher the probability of ultramicroscopic airborne liquid mist being generated, and therefore, the image quality becomes worse due to the adhesion of the liquid mist to the recording surface of a sheet of recording medium.
Thus, the primary object of the present invention is to provide a liquid ejection method that uses a liquid ejection head capable of ejecting extremely small liquid droplets, and in which a bubble connects to the atmospheric air, in such a way that liquid droplets are ejected without deviating from the predetermined ejection direction, thereby accomplishing high quality recording.
Another object of the present invention is to provide a liquid ejection method which does not allow liquid mist to be generated even when liquid droplets are reduced extremely in volume in order to increase image quality.
The present invention was made as an innovative liquid ejection method based on the bubble-air connection system, and was discovered during the research and development carried out to solve the aforementioned problems in the liquid ejection methods based on the bubble-air connection system which had been disclosed earlier. The knowledge acquired by the inventors of the present invention during the research and development carried out in order to accomplish the aforementioned objects are as follows.
The present invention was made by paying attention to the fact that the formation of a bubble by heat is an extremely stable process, but if the volume of a liquid droplet is reduced enough to achieve a high quality image, even an extremely small amount of change to a bubble is not insignificant. Furthermore, a small amount of xe2x80x9cwettingxe2x80x9d which is caused by ink droplets adjacent to ejection orifices is not insignificant in terms of the direction in which liquid droplets are ejected. Prior to the aforementioned research and development conducted by the inventors of the present invention, attention had been paid only to the process in which a bubble becomes connected to the atmospheric air, whereas the present invention pays attention to a process subsequent to the bubble connecting to the atmospheric air, as well as to the connecting process.
The essence of the present invention, which is based on the above-described knowledge, is as follows.
The present invention is characterized in that in a liquid ejection method, which employs a liquid ejection head comprising electrothermal transducers for generating thermal energy for ejecting liquid, liquid ejection orifices positioned so as to face, one for one, the electrothermal transducers, and liquid paths which lead, one for one, to the liquid ejection orifices, delivering liquid to the ejection orifices, and in which each of the electrothermal transducers is disposed on the bottom surface and ejects the liquid with the use of the pressure of a bubble generated through a process in which the liquid in the liquid path is caused to undergo a change of state by the application of thermal energy to the liquid, the generated bubble is allowed to become connected to the atmospheric air only after the bubble begins to reduce in volume after it grows to its maximum volume.
Furthermore, the present invention is characterized in that a liquid ejection method, which employs a liquid ejection head comprising electrothermal transducers for generating thermal energy for ejecting liquid, liquid ejection orifices positioned so as to face, one for one, the electrothermal transducers, and liquid paths which lead, one for one, to the liquid ejection orifices, delivering liquid to the ejection orifices, and in which each of the electrothermal transducers is disposed on the bottom surface, and ejects the liquid with the use of the pressure of a bubble generated through a process in which the liquid in the liquid path is caused to undergo a change of state by the application of thermal energy to the liquid, comprises a process in which atmospheric air is introduced into the liquid path to which the bubble becomes connected, a process in which the liquid reaches the electrothermal transducers after the introduction of the atmospheric air into the liquid path, and a process in which a small amount of the liquid in the liquid path is separated from the liquid in the liquid path and forms a liquid droplet.
Furthermore, the present invention is characterized in that in a liquid ejection method, which employs a liquid ejection head comprising electrothermal transducers for generating thermal energy for ejecting liquid, liquid ejection orifices, positioned so as to face, one for one, the electrothermal transducers, and liquid paths which lead, one for one, to the liquid ejection orifices, delivering liquid to the ejection orifices, and in which each of the electrothermal transducers is disposed on the bottom surface., and ejects the liquid with the use of the pressure of a bubble generated through a process in which the liquid in the liquid path is caused to undergo a change of state by the application of thermal energy to the liquid, the liquid which is in the liquid path and which covers the electrothermal transducer in the liquid path is separated by a small portion, and becomes a liquid droplet, at the same time as the bubble becomes connected to the atmospheric air and the atmospheric air is introduced into the liquid path.
Further, the present invention is characterized in that in a liquid ejection method, which employs a liquid ejection head comprising electrothermal transducers for generating thermal energy for ejecting liquid, liquid ejection orifices positioned so as to face, one for one, the electrothermal transducers, and liquid paths which lead, one for one, to the liquid ejection orifices, delivering liquid to the ejection orifices, and in which the each of electrothermal transducers is disposed on the bottom surface, and ejects the liquid with the use of the pressure of a bubble generated through a process, in which the liquid in the liquid path is caused by undergo a change of state by the application of thermal energy to the liquid, the liquid is ejected as the bubble becomes connected to the atmospheric air after the growth speed of the bubble becomes negative.
According to any of the liquid ejection head structures described above, a bubble is allowed to become connected to the atmospheric air only after the bubble begins to decrease in volume. Therefore, in the process in which a primary liquid droplet is formed, the portion of the liquid which is immediately adjacent to the top portion of the bubble and extends downward (toward the electrothermal transducer) from the primary droplet portion of the liquid, and which, if ejected, will form satellite liquid droplets that are the source of the splashing which occurs during the liquid ejection, can be separated from the primary droplet portion. Therefore, the amount of mist is substantially reduced, which in turn considerably reduces the amount of the soiling which occurs to the recording surface of a sheet of recording medium due to the mist. Further, the portion of the liquid which will form satellite ink droplets if ejected is dropped onto, or caused to adhere to, the electrothermal transducer. After dropping onto, or adhering to, the electrothermal transducer, this portion of the liquid possesses a vector that is parallel to the surface of the electrothermal transducer, and therefore, this portion, that is, the potential satellite droplet portion, is easily separated from the primary droplet portion of the liquid. Therefore, as described above, the amount of the mist is substantially reduced, which in turn considerably reduces the amount of the soiling which occurs to the recording surface of a sheet of recording medium due to the mist. Furthermore, according to the above-described structure, the point at which the primary droplet portion of the liquid is separated from the rest of the liquid aligns with the central axis of the ejection hole, and therefore, the direction in which the liquid is ejected is stabilized. In other words, the liquid is always ejected in the direction substantially perpendicular to the surface of the electrothermal transducer, that is, the liquid ejecting surface of the head. As a result, it is possible to record a high-quality image which does not suffer from the problems traceable to the deviation due to the liquid ejection direction.
Whether a bubble becomes connected to the atmospheric air during its growth or during its contraction depends on the geometric factors of the liquid path and the ejection orifice, the size of the electrothermal transducer, and also the properties of the recording liquid.
More specifically, if the flow resistance of a liquid path (between electrothermal transducer and liquid supply path) is low, it is easier for a bubble to grow toward the liquid supply path, which reduces the bubble growth speed toward an ejection orifice. Thus, the connection between a bubble and the atmospheric air is more likely to occur during the contraction of the bubble. If a plate (hereinafter xe2x80x9corifice platexe2x80x9d) through which ejection holes are formed is increased in thickness, the viscosity resistance of the recording liquid during bubble growth increases, and therefore, the connection between a bubble and the atmospheric air is more likely to occur during the contraction of the bubble. Furthermore, a thicker orifice plate stabilizes a liquid ejection head in terms of liquid ejection direction, and therefore, the smaller the deviation in liquid ejection direction. This also makes a thicker orifice plate more desirable. If an electrothermal transducer is excessively large, the connection between a bubble and the atmospheric air is more liable to occur during the growth of the bubble. Therefore, attention must be paid to the electrothermal transducer size. Furthermore, if the recording liquid viscosity is excessively high, the connection between a bubble and the atmospheric air is more likely to occur during the contraction of the bubble.
Furthermore, the way a bubble becomes connected to the atmospheric air changes depending on the cross-section of the ejection hole in an orifice plate, which cross-section is perpendicular to the axis of the hole. More specifically, assuming that an ejection orifice diameter remains the same, the greater the angle of the taper of the ejection hole wall in the cross section (the smaller the orifice diameter relative to the diameter of the bottom opening of the ejection hole), the more likely the connection between a bubble and the atmospheric air will occur during the contraction of the bubble.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.